1. Field of the Invention
The present invention generally relates to an operation test for determining the duty ratio of drive voltage applied to a steering mechanism in an automatic steering device.
2. Background Information
A ship that is underway is subjected to the effects of waves, wind, and other such external disturbances, which cause the orientation of the ship to fluctuate constantly. Therefore, small steering inputs have to be made continuously in order to maintain a steady course (the direction in which the prow is facing), and this imposes a burden on the helmsman. There are known automatic steering devices that provide feedback control of the steering mechanism so as to maintain a preset course automatically. An automatic steering device such as this is discussed in Japanese Laid-Open Patent Application Publication No. H1-289797 (Patent Literature 1), for example.
The amount by which a steering mechanism changes the rudder angle (the angle of the rudder) per unit of time is called the rudder turning speed. If the rudder turning speed is too slow, the vessel is less maneuverable, but if the rudder turning speed is too fast, the vessel becomes unstable. In view of this, an automatic steering device preferably controls the steering mechanism so as to move the rudder at the optimal rudder turning speed.
The optimal rudder turning speed varies with the vessel speed and hull length. FIG. 2 is a graph of how hull length and cruising speed are related to the range of the optimal rudder turning speed. The term cruising speed refers to the speed at which the vessel moves in a state of the best fuel economy, and is the usual movement speed of a vessel. The hull length and cruising speed are determined for each vessel, so the optimal rudder turning speed is predetermined for each vessel. Therefore, an automatic steering device should control the steering mechanism so as to achieve the predetermined optimal rudder turning speed.
Patent Literature 1, Japanese Laid-Open Patent Application Publication No. 2004-217180 (Patent Literature 2), and Japanese Laid-Open Patent Application Publication No. S59-81274 (Patent Literature 3) disclose constitutions in which the rudder turning speed is varied according to vessel speed (or vehicle speed). Thus, the rudder turning speed may be varied according to the situation. In any case, it is desirable for an automatic steering device to control a steering mechanism properly so as to achieve a desired rudder turning speed.
The drive mechanism (a hydraulic pump, a motor, etc.) of a steering mechanism is usually controlled by PWM (pulse width modulation) method. In this case, the automatic steering device can control the rudder turning speed with the steering mechanism by setting the duty ratio of the drive voltage outputted to the steering mechanism. Since individual steering mechanisms vary from one to another, the duty ratio that is required to achieve the optimal rudder turning speed will vary from one steering mechanism to the next. In view of this, in the past, when an automatic steering device is initially installed in a vessel, an operation test (called a “rudder test”) is performed to measure the rudder turning speed by driving the steering mechanism at a specific duty ratio.
A conventional operation test method will be described through reference to the flowchart in FIG. 6. In this operation test, the steering mechanism is driven while lowering the duty ratio in steps, and the rudder turning speed during this time is measured, in order to find the duty ratio at which the optimal rudder turning speed can be attained.
When an automatic steering device is initially installed in a vessel, the user starts the operation test by first subjecting this automatic steering device to suitable operations. Once the operation test is started by the user, the automatic steering device sets the duty ratio of the drive voltage outputted to the steering mechanism to the maximum value (100%) (step S201). The automatic steering device then applies the drive voltage of the set duty ratio to the steering mechanism, thereby driving the steering mechanism (step S202). This moves the rudder and changes the rudder angle. The automatic steering device measures the rudder turning speed by using a suitable sensor or the like to detect this movement of the rudder (step S203).
Then, the automatic steering device determines whether or not the measured rudder turning speed falls within the optimal range for rudder turning speed illustrated in FIG. 2 (step S204). If the rudder turning speed does fall within the optimal range for rudder turning speed, the automatic steering device stores the duty ratio used for measuring this rudder turning speed as the optimal duty ratio (step S205), and displays the resulting optimal duty ratio on a suitable display means (step S206).
If the measured rudder turning speed does not fall within the optimal range in the determination in step S204, the automatic steering device reduces the duty ratio in steps and repeats the above-mentioned measurement of rudder turning speed. For example, in the flowchart in FIG. 6, the duty ratio is lowered by 10% each time (step S208), and measurement of the rudder turning speed is repeated until the rudder turning speed falls within the optimal range.
Because a certain amount of force is necessary to move a rudder, the rudder may not move if the duty ratio is too low. Therefore, there is no point in performing the above operation test at a duty ratio setting that is too low. In view of this, in the flowchart in FIG. 6 the test is stopped when the duty ratio drops below 50% (step S207).
The above operation test can find the optimal duty ratio for achieving the optimal rudder turning speed at cruising speed. In automatic steering control by an automatic steering device, the steering mechanism is driven by applying drive voltage of the optimal duty ratio found in the above operation test to the steering mechanism. This allows the automatic steering device to perform automatic steering control at the optimal rudder turning speed at cruising speed.